Electrochemical devices for separating oxygen from air all utilize one or more electrochemical air separation elements formed of an electrolyte layer sandwiched between one or more electrode layers. Such devices include oxygen concentrators, hydrolyzers and fuel cells.
The electrolyte is capable of conducting oxygen ions at elevated temperatures. Typical materials used in forming the electrolyte include yttria stabilized zirconia and gadolinium doped ceria. The electrode layers serve to conduct electrons and act as a cathode and an anode for the air separation element. The cathode electrode conducts electrons to ionize the oxygen and the anode electrode conducts electrons that are produced by recombination of the oxygen ions into elemental oxygen. In case of an oxygen concentrator or hydrolyzer, the oxygen ion transport is driven by an external electric current impressed on the electrodes. In a fuel cell, the oxygen ion transport is driven by an oxygen partial pressure differential produced by the combustion of fuel supported by the permeated oxygen. The electrode layers are porous to permit oxygen diffusion to and from the electrolyte and are typically fabricated from metals, metallic oxides or mixtures of the same.
In order to connect the electrode to an external current source in case of an oxygen concentrator or to a load in case of fuel cell, porous current collector layers are disposed on the electrode layers. The current collector distributes current evenly throughout the electrode surface so that the entire electrode surface is active. A typical material that is used for a current collector is silver. The current collector layers are applied to the electrode layers in a green state by a variety of different methods including slurry dip coating, spraying and isopressing. The green form is subjected to a heat treatment in which additives such as binders and fugitive pore formers, when used, are burned out and the silver particles partly sinter into a coherent mass so that the coherent mass has a porous structure. Typically, the yield of pores is low due to the sintering because as the sintering process proceeds any pores that have been formed are eliminated due to densification of the mass. Moreover, during use of the electrochemical air separation element, further pore closure occurs. As can be appreciated, the closure of the pores decreases the performance of the element because the oxygen has to diffuse through the current collector rather than through pores formed in the current collector. Furthermore, the conductive metal, for instance silver, tends to evaporate during use which decreases longevity of the device. A yet further problem is that it is difficult to maintain the current collector affixed to the electrode for any length of time.
In an attempt to solve problems such as pore closure and aging due to evaporation, U.S. Pat. No. 6,457,657 discloses a method of fabricating a current collector that employs an even mixture of a conductive metal, for example, silver and a metallic oxide, for instance, an 8 percent yttria stabilized zirconia. The metallic oxide tends to prevent the closure of the pores. A layer of a metallic oxide is added over the current collector to prevent aging of the device due to evaporation of the silver. The problem with a current collector formed by an even mix of a conductor and a metallic oxide is that the metallic phase, for instance, silver, will tend to more readily sinter than the metallic oxide. As a result, a high proportion of the silver is exposed and will have a high degree of interparticle contact to decrease the yield of pores during manufacture and the maintenance of pores during operational use. Furthermore, the high concentrations of metallic oxides tend to reduce the conductivity of the current collector.
As will be discussed, the present invention provides a method of manufacturing a current collector that utilizes metallic oxides in a more effective manner than the prior art to increase the yield and longevity of open pores and that also increases the adhesion of the current collector to an electrode.